Initialize the frequeny once and use it, avoid calling a native method every

sdk/lib/core/stopwatch.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * A simple stopwatch interface to measure elapsed time.
  */
 class Stopwatch {
+  /**
+   * Frequency of the elapsed counter in Hz.
+   */
+  final int frequency = _frequency();
+
   // The _start and _stop fields capture the time when [start] and [stop]
   // are called respectively.
   // If _start is null, then the [Stopwatch] has not been started yet.
   // If _stop is null, then the [Stopwatch] has not been stopped yet,
   // or is running.
   int _start;
   int _stop;
 
   /**
    * Creates a [Stopwatch] in stopped state with a zero elapsed count.
    *
    * The following example shows how to start a [Stopwatch]
    * immediately after allocation.
    *
    *     Stopwatch stopwatch = new Stopwatch()..start();
    */
-  Stopwatch() : _start = null, _stop = null {}
+  Stopwatch();
 
   /**
    * Starts the [Stopwatch].
    *
    * The [elapsed] count is increasing monotonically. If the [Stopwatch] has
    * been stopped, then calling start again restarts it without resetting the
    * [elapsed] count.
    *
    * If the [Stopwatch] is currently running, then calling start does nothing.
    */
@@ skipping 71 matching lines @@
     return (elapsedTicks * 1000000) ~/ frequency;
   }
 
   /**
    * Returns the [elapsedTicks] counter converted to milliseconds.
    */
   int get elapsedMilliseconds {
     return (elapsedTicks * 1000) ~/ frequency;
   }
 
-  /**
-   * Returns the frequency of the elapsed counter in Hz.
-   */
-  int get frequency => _frequency();
 
   /**
    * Returns wether the [StopWatch] is currently running.
    */
   bool get isRunning => _start != null && _stop == null;
 
   external static int _frequency();
   external static int _now();
 }